GRAINS CAN EXPLODE: DUST EXPLOSION

DUST EXPLOSION

An explosion is defined as a sudden reaction involving a rapid physical or chemical oxidation reaction or decay generating an increase in temperature or pressure or both simultaneously. The most familiar reactions are those of flammable gases, vapours or dust with the oxygen contained in the air. Dust explosions have been known in Europe since the first cereal mills were built. There is a risk of fires and explosions in all industrial plants which have to deal with combustible dusts, whether this be as production material or as residual waste.

MECHANISM OF EXPLOSION:

Four ingredients are required for a grain dust explosion:

1. Oxygen,
2. Ignition source,
3. Fuel
4. Containment.

Grain dust explosions are in reality a series of explosions. The fuel for a dust explosion is the grain dust in suspension in the air at or above the minimum explosive concentration (MEC). The first explosion referred to as the "primary explosion" is usually small with pressures less than 2 pounds per square inch, (psi). Primary wave propagates a pressure wave and a fire front.  The pressure wave moves away from the location of the primary explosion at a speed of about 1000 feet per second (fps). The fire front follows at the speed of about 10 fps. The movement of the pressure wave results in secondary concentrations that are subsequently ignited by the relatively slow moving fire front. Secondary explosions can result in rupture pressures in excess of 100 psi. Experts use 50 grams per cubic meter as the MEC for grain dust. If the dust concentration at a grain transfer point is less than the MEC an explosion will not occur even if all other ingredients are present.

Sources of explosion:

Strehlow [1982] found two major sources of dust explosions:

1.  The bucket elevator, considered to be the most dangerous piece of equipment in the elevator.
2. Poor housekeeping practices that allowed accumulation of dust provided the fuel for an explosion.  A heat-generated fire or a spark provided the ignition source for an explosion.

The Mount Mulligan mine disaster occurred on September 19, 1921 in Mount Mulligan, Far North Queensland, Australia. A series of explosions in the local coal mine, audible as much as 30 km away, rocked the close knit township.

Seventy-five workers were killed by the disaster which is the third worst coal mining accident in Australia in terms of human lives lost. Four of the dead had been at the mouth of the pit at the time of the explosion. Only eleven of the bodies were found. The disaster affected people in cities and towns all over the country. The mine was new at the time of the accident and it was widely considered as safe without previous indications of gas leaks. The miners hence worked using open flame lights instead of safety lamps.

 Preventive measure of dust explosion:

1.      Prevention of grain dust explosions is associated with eliminating one or more of the ingredients, other than oxygen, associated with dust explosions.

2.      Removal of ignition sources eliminates "ignition".

3.      Pneumatic dust control and dust suppression using oil or water additives reduces the concentration of grain dust suspended in air at a grain transfer point.

Explosion venting is used to vent the products of combustion from the primary ignition prior to ignition of secondary explosions.